What Is a Phenylketonuria Test?

Part of the general screening for newborn diseases, a phenylketonuria (PKU) test looks for one specific genetic disease. This procedure involves a heel-prick blood sample, which is then tested for unusual levels of a substance called phenylalanine. An abnormal phenylketonuria test result might mean that the baby has phenylketonuria disease, which can cause mental retardation if a special diet is not adopted for life.

Phenylketonuria is a rare disease. Babies who have the condition have an abnormal copy of a gene that produces a particular enzyme. In healthy infants, the enzyme breaks down phenylalanine, an amino acid that makes up part of many proteins. When proteins are ingested as food, the body normally breaks them down properly and uses them safely.

In kids who have PKU, the enzyme is not capable of breaking down phenylalanine. The excess buildup of the amino acid in the body causes brain damage and other symptoms, such as eczema and seizures. PKU results entirely from the inability of the body to break down phenylalanine from food, so newborn babies show no symptoms because they have not yet ingested any food.

This is why the phenylketonuria test is not administered to babies before they have had their first feed. Instead, the blood sample is taken when the infant is 24-72 hours old, when they have had a chance to attempt to metabolize the amino acid in milk. Even though the condition is rare, in many countries, including the United States, the test is offered to all newborns. An early enough test can allow the doctor to diagnose the presence of a mutated gene. He or she can then place the child on a special diet, restricting phenylalanine sources, which prevents the development of any PKU illness and allows the child to develop normally.

For the phenylketonuria test, a nurse disinfects part of the newborn's heel. He or she then pricks the clean skin with a lancet so that blood spots appear. The nurse dabs a piece of paper three times on the blood spot, forming three circles of blood on the paper.

The baby's foot might stop bleeding by itself, or the nurse might bandage up the area. Sometimes the nurse might choose to take the samples from the inside of the baby's elbow instead of the heel. The baby might cry during the procedure, and the nurse might offer the infant sugar water to soothe it. The risks of the procedure are low, but infection, blood blisters and fainting are possibilities.

A hospital laboratory then tests the blood on the paper for the presence of phenylalanine. An analyst prepares a Petri dish that contains an agar medium with the substance B-2-thienylalanine in it. The agar is covered with a layer of a bacterium such as Bacillus subtilis. The B-2-thienylalanine chemical prevents the bacteria from growing on the plate.

In the absence of phenylalanine, no bacteria grow on the plate. When phenylalanine is added to the chemical and the bacteria, however, the phenylalanine blocks the inhibitory action of the chemical and allows the bacteria to grow. Therefore, when a paper disc that is covered in blood with phenylalanine is placed on the plate, bacteria will grow only under the disc and not on the rest of the plate. This is a positive result for PKU.

Another form of phenylketonuria test involves genetic testing. A doctor can sample cells from the amniotic fluid surrounding a fetus and send them to a genetics laboratory to test for the presence of a mutated gene. This type of test might be useful for women who themselves have PKU or whose partners have PKU.